This invention relates to a distillate material having a high cetane number and useful as a diesel fuel or as a blending stock therefor, as well as the process for preparing the distillate. More particularly, this invention relates to a process for preparing distillate from a Fischer-Tropsch wax.
Clean distillates that contain no or nil sulfur, nitrogen, or aromatics, are, or will likely be in great demand as diesel fuel or in blending diesel fuel. Clean distillates having relatively high cetane number are particularly valuable. Typical petroleum derived distillates are not clean, in that they typically contain significant amounts of sulfur, nitrogen, and aromatics, and they have relatively low cetane numbers. Clean distillates can be produced from petroleum based distillates through severe hydrotreating at great expense. Such severe hydrotreating imparts relatively little improvement in cetane number and also adversely impacts the fuel""s lubricity. Fuel lubricity, required for the efficient operation of fuel delivery system, can be improved by the use of costly additive packages. The production of clean, high cetane number distillates from Fischer-Tropsch waxes has been discussed in the open literature, but the processes disclosed for preparing such distillates also leave the distillate lacking in one or more important properties, e.g., lubricity. The Fischer-Tropsch distillates disclosed, therefore, require blending with other less desirable stocks or the use of costly additives. These earlier schemes disclose hydrotreating the total Fischer-Tropsch product, including the entire 700xc2x0 F.xe2x88x92 fraction. This hydro-treating results in the elimination of oxygenates from the distillate.
By virtue of this present invention small amounts of oxygenates are retained, the resulting product having both very high cetane number and high lubricity. This product is therefore useful as a diesel fuel as such, or as a blending stock for preparing diesel fuels from other lower grade material.
In accordance with this invention, a clean distillate useful as a fuel heavier than gasoline, e.g., useful as a diesel fuel or as a diesel fuel blend stock and having a cetane number of at least about 60, preferably at least about 70, more preferably at least about 74, is produced, preferably from a Fischer-Tropsch wax and preferably derived from a cobalt or ruthenium Fischer-Tropsch catalyst, by separating the waxy product into a heavier fraction and a lighter fraction. The nominal separation is at about 700xc2x0 F., and the heavier fraction contains primarily 700xc2x0 F.+, and the lighter fraction contains primarily 700xc2x0 F.xe2x88x92.
The heavier fraction is subjected to hydroisomerization in the presence of a hydroisomerization catalyst, having one or more noble or non-noble metals, at normal hydroisomerization conditions, where at least a portion of the 700xc2x0 F.+ material is converted to 700xc2x0 F.xe2x88x92 material. At least a portion and preferably all of the lighter fraction, preferably after separation of C5xe2x88x92 (although some C3 and C4 may be dissolved in the C5+) remains untreated, i.e., other than by physical separation, and is blended back with at least a portion and preferably all of the hydroisomerized, 700xc2x0 F.xe2x88x92, product. From this combined product a diesel fuel or diesel blending stock in the boiling range 250xc2x0 F.-700xc2x0 F. can be recovered and has the properties described below.